Lead-acid accumulator alloy



United States Patent 2,860,969 LEAD-ACID ACCUMULATOR ALLOY Stanley Walsh, Walkden, near Manchester, England, as-

signor to The Chloride Electrical Storage Company Limited, London, England, a British company No Drawing. Application January 31, 1957 Serial No. 637,359

Claims priority, application Great Britain April 26, 1956 5 Claims. (Cl. 75-167) The grids of the plates of lead-acid accumulators generally consist of cast lead-antimony alloys. Antimony is added to the lead mainly to provide adequate mechanical strength, resistance to creep, and hardness. Unfortunately, antimony, although it improves the physical characteristics of the grid metal, is gradually leached out through oxidation and has certain electrochemical disadvantages in the cell. In particular owing to the difference between the electrode potential of antimony and lead, antimony when dissolved in the electrolyte is electro-deposited on the surface of the negative plate and sets up a chemical reaction with the reduced spongy lead. This leads to the formation of lead sulphate, causing the n'egative plate to become discharged when the battery is standing on open circuit. The elimination of antimony from an accumulator grid alloy is therefore a matter of considerable importance; various other lead-based alloys not containing antimony have been proposed to overcome this difiiculty. One group of such proposed alloys, is that containing calcium in the range of 0.03 to 0.10% with or without other alloying elements such as tin, silver and aluminium. Lead based alloys of calcium, tin and aluminium are referred to in my Patent No. 2,794,707. These alloys after a short ageing period after casting reach a state of hardness and tensile strength equivalent to those of the lead antimony alloys in common use. Lead-alloys containing calcium have a large grain structure, and when tin is present under the working conditions during the cycling of the positive plates of an accumulator, they are subject to severe intergranular corrosion which causes disintegration of the grid and premature failure of the cell. When tin is not present, the leadbased calcium alloys suffer from excessive growth which also leads to premature failure during deep cycling.

In accordance with the present invention, to lead alloys containing calcium, with or without tin, silver and aluminium, are added small amounts of the rare earth metal cerium, which is found to have the eifect of cansing a marked reduction in the grain size of the alloy. In alloys containing tin as well as calcium, the addition of cerium greatly enhances the resistance to the effect of intergranular attack, resulting from anodic corrosion. The cerium can be added directly to the molten lead calcium alloys as the element itself, or in the formof the metal referred to as mischmetal which contains a relatively high proportion of cerium. It is found that under the normal casting conditions for accumulator grids, a small addition of cerium or mischmetal, in the range 0.01 to 0.1%, produces an alloy having a fine equi-axed grain structure. The grain refinement is successful if carried out with lead-calcium, lead-tin-calcium, lead-tinsilver-calcium alloys, and also if these alloys contain a small amount of aluminium.

The following are examples of the improved alloys with refined grain structures:

2 Percent Aluminium 0.01-0.10 Cerium 0.01-0.10 Calcium 0.03-0.10 Lead Residue Aluminium 0.01-0.10 Cerium 0.01-0.10 Calcium 0.03-0.10 Tin 1-2 Lead Residue Aluminium 0.01-0.10 Cerium 0.01-1.10 Calcium 0.03-0.10 Tin 0.4-0.6 Silver 0.1-0.5 Lead Residue Cerium 0.01-0.10 Calcium 0.03-0.10 Lead Residue Cerium 0.01-0.10 Calcium 0.03-0.10 Tin 1-2 Lead Residue Cerium 0.01-0.10 Calcium 0.03-0.10 Tin 0.4-0.6 Silver 0.1-0.5 Lead Residue These alloys have at least 30 grains across a diameter casting and the grains are equi-axed. Similar alloys without the addition of cerium or rare earth metals have less than 10 grains across a diameter casting, and some of the grains may be columnar.

What I claim is:

1. A lead-based calcium alloy containing calcium from 0.030.10% and cerium from 0.01-0.10% and having an equi-axed grain structure.

2. A lead-based calcium alloy containing calcium from 0.03-0.10% and cerium from 0.01-0.10% and having an equi-axed grain structure and at least 30 grains across a diameter casting.

3. The alloy of claim 1, containing 0.01-0.1% of aluminum.

4. The alloy of claim 1, containing 1-2% of tin.

5. The alloy of-claim 1, containing 0.1-0.5% of silver.

References Cited in the file of this patent UNITED STATES PATENTS 2,042,840 Haring June 2, 1936 2,159,124 Betterton et al May 23, 1939 2,170,650 Bonton Aug. 22, 1939 2,588,095 Eckel Mar. 4, 1952 2,794,707 Walsh June 4, 1957 FOREIGN PATENTS 140,790 Great Britain Nov. 11, 1920 579,380 Great Britain Aug. 1, 1946 

2. A LEAD-BASED CALCIUM ALLOY CONTAINING CALCIUM FROM 0.03-0.10% AND CERIUM FROM 0.01-0.10% AND HAVING AN EQUI-AXED GRAIN STRUCTURE AND AT LEAST 30 GRAINS ACROSS A 3/8" DIAMETER CASTING. 